Iron and steel parts such as blooms or billets, before rolling, are heated in generally continuous furnaces. In these furnaces, the iron and steel products are very often supported by cooled tubes surrounded by a lagging called "fire dogs", via cylindrical supports fixed to the fire dogs with even spacing and called studs or buttons.
These fire dogs are fixed in blast furnaces. In so called "walking beam" furnaces, half of the fire dogs may be moved in a vertical plane, with a circular or rectangular movement; this movement causes the products to advance in the furnace, by successive steps. In this type of furnace, these fire dogs up to present have always had their axis parallel to the longitudinal axis of the furnace.
The fire dogs are the cause of heating defects due to the shadow effect of the blocked radiation of the furnace on the supported face of the products. To this shadow effect is added the pin point cooling due to the contact of these products with the studs which support them directly.
Such shadow effect and pin point cooling generate lower temperature zones called "blank traces" on the products which then cause disturbances during rolling.
It is known to reduce these blank traces by slanting or offsetting the fire dogs with respect to the axis of the furnace so as to expose all the parts of the supported face of the products to the radiation for a length of time substantially equal in the same intervals.
When it is a question of a walking beam furnace, this condition is more difficult to achieve because of the successive passages of the product over the fixed fire dogs and the mobile fire dogs which means that the different parts of the supported surface are not exposed in the same way and do not receive the same amounts of radiation during the passage through the furnace.